The recent ever-increasing supply of scrap iron and steel has posed an important problem with its recycling for the effective use of resources. This has stimulated the development of a new technology for the efficient, economical production of molten iron from scrap.
It has been common practice to use an electric furnace for the production of molten iron from scrap. This process is uneconomical because of heavy consumption of electric energy.
Another way of producing molten iron (for foundry) is by the cupola process. This process suffers the disadvantage of requiring high-quality foundry lump coke as fuel, which is about four times as expensive as blast furnace coke, and hence is not in general use. The necessity for special foundry coke (which is coarser and less reactive and combustible than blast furnace coke) is due to the fact that combustion to promote the smooth melting of scrap in the cupola furnace should take place such that oxygen in the hot air blown from the tuyere is not rapidly cooled by coke at the raceway but is consumed near the scrap melting zone above the coke bed where the temperature reaches a maximum.
As a substitute for the conventional electric furnace process or cupola process mentioned above, there has been proposed a scrap melting process in "Tetsu-to-Hagane", vol. 79, No. 2, pp.139-146. According to this process, scrap melting is accomplished in a shaft furnace charged with scrap (as an iron source) and blast furnace coke which is burned by oxygen-enriched air (at normal temperature) blown together with pulverized coal through the tuyere. The resulting combustion gas produces sensible heat necessary for scrap melting, and scrap melting is promoted by the secondary combustion of the combustion gas which is induced by air blown through the shaft.
There is another scrap melting process as disclosed in Japanese Patent Laid-open No. 195225/1989. This process resorts to a shaft furnace and a separate combustion furnace in which a large amount of pulverized coal is burned. The resulting hot combustion gas is introduced into the shaft furnace charged with scrap and coke. At the same time, an oxygen-containing gas is supplied for the secondary combustion of the combustion gas which generates sensible heat necessary for scrap melting.
The foregoing two scrap melting processes are likely to be economical because they employ pulverized coal (as part of heat source) and inexpensive blast furnace coke to be charged into the furnace.
However, they are merely intended to save energy by reducing the fuel ratio (below 300 kg/t-pig). This object is achieved by blowing air (or oxygen-containing gas) into the combustion gas resulting from the combustion of pulverized coal so as to bring about the secondary combustion. In other words, their aim is a cost reduction through the reduction of fuel ratio and the use of pulverized coal as part of heat source. In fact, their aim differs from that of operating at a high fuel ratio with a large amount of pulverized coal to be intentionally converted into a large amount of combustion gas (or exhaust gas). They are not designed for operation under such conditions.
In addition, although the above-mentioned scrap melting processes employ pulverized coal as part of heat source for cost reduction, they do not fully achieve this object in view of the fact the fuel ratio is low but the coke ratio is high. It should be noted that the ratio (by weight) of pulverized coal to coke is lower than 1.0 (or 0.9 at the highest).
Another disadvantage of these conventional scrap melting processes is that the exhaust gas inevitably contains a large amount of nitrogen and carbon dioxide because air (as an oxygen-containing gas) is blown to effect the secondary combustion of the combustion gas of pulverized coal for the reduction of fuel ratio and air or oxygen-enriched air is used for the combustion of pulverized coal and the secondary combustion. Although such an exhaust gas is valuable as a fuel gas in its own way, it is not a high-calorie fuel gas suitable for efficient power generation or heating furnaces.
Regarding the calorific value of exhaust gas, the above-cited literature mentions that, by contrast with the cupola process, the proposed process yields a high-calorie exhaust gas that can be effectively utilized as a fuel gas. The fact is that the calorific value of the exhaust gas is only 2000 kcal/Nm.sup.3 (or 8400 kJ/Nm.sup.3) or so. The same literature gives experimental data obtained in the test run without secondary combustion; however, the present inventors' trial calculations indicate that the calorific value of the exhaust gas is 2300 kcal/Nm.sup.3 at the highest. In other words, the exhaust gas obtained in the conventional process is not suitable for heating furnaces and efficient power generation and hence is not so valuable in view of the fact the fuel gas used for heating furnaces and efficient power generation has a calorific value higher than 2500 kcal/Nm.sup.3. In addition, the conventional process, which is designed for operation at a low fuel ratio and hence emits a limited amount of exhaust gas with a low calorific value, cannot be a stable supply source for a large amount of high-quality fuel gas.
The second conventional process (proposed in the above-cited Japanese Patent) suffers the disadvantage of requiring a combustion furnace for the combustion of pulverized coal separately from the melting furnace. This leads to a high installation cost. This also poses an economical problem with partial loss of heat which occurs while the hot gas is being introduced from the combustion furnace to the shaft furnace through a gas duct.
An improvement on the above-mentioned cupola process has been proposed which involves blowing oxygen-enriched hot air together with pulverized coal from the tuyere. (Klaus Scheiding: Proceedings of the Eighth Japan-Germany Seminar, Oct. 6, 7, 1993 (Sendai, Japan), p. 22, "Hot Metal Production Based on Scrap, Coal and Oxygen") This improved cupola process suffers the disadvantage of requiring blast furnace coke of large size, which leads to high production cost. Like the above-mentioned prior art technologies, this improved cupola process is not intended to produce fuel gas by supplying a large amount of pulverized coal, nor is it designed for operation under such conditions. Moreover, the fact that it involves the blowing of hot air (which contains nitrogen) suggests the impossibility of producing high-calorie exhaust gas.
As mentioned above, the conventional scrap melting processes proposed so far are basically intended to save energy by the reduction of fuel ratio. Therefore, they merely give rise to an exhaust gas which is limited in calorific value and amount and hence of little economical value. In addition, they need pulverized coal as part of the heat source but the ratio of pulverized coal to coke is not sufficiently high because of incapability of efficient combustion of pulverized coal. This means that they do not make best use of pulverized coal for cost reduction.
In the meantime, the ever-increasing waste plastics as industrial waste and municipal waste has recently posed a serious problem with their disposal. Waste plastics (of polymeric hydrocarbons) are disposed of mostly by dumping on a reclaimed land because they cannot be disposed of by combustion. (They generate so much heat as to damage the incinerator during combustion.) Dumping of waste plastics is not desirable from the environmental point of view. Thus there is a demand for the development of a method for disposing of a large amount of waste plastics.
The problem with waste disposal is involved also in the steel industry. That is, the so-called integrated steel mill emits a large amount of dust, including blast furnace dust, converter dust, electric furnace dust, cupola dust, mill scale, shredder dust, and zinc dust. They contain zinc in comparatively high concentrations (1-2% for blast furnace dust and about 20% for cupola dust), and hence they cannot be disposed of by dumping on a reclaimed land for environmental protection. Thus there is a demand for the development of a process for their disposal.
A process for disposing of zinc-containing dust has been proposed in Japanese Patent Laid-open Nos. 25221/1978 and 125211/1980. This process consists of pelletizing zinc-containing dust, charging the pellets into a shaft furnace, reducing and evaporating zinc in the furnace, oxidizing zinc in the waste gas, and collecting the resulting zinc oxide. There is another process proposed in Japanese Patent Laid-open No. 263088/1990. According to this process, metal-containing powder (such as zinc dust emitted from a cupola) is repeatedly introduced into the cupola through the tuyere so that zinc in the dust is concentrated.
The first process mentioned above is not suitable costwise for disposal of a large amount of dust because of its necessity for pelletizing zinc-containing dust.
In addition, both the first and second processes mentioned above suffer the disadvantage that zinc vapor in the furnace condenses on the furnace wall before it reaches the furnace top, thereby causing the refractories to peel off. Such condensation occurs because the temperature at the top of a blast furnace or cupola is about 200-250.degree. C. and the temperature of the shaft is about 400-800.degree. C. at which zinc vapor condenses in the furnace.
The scrap melting process also involves a problem of avoiding the accumulation of zinc in the furnace and recovering zinc adequately because scrap (as the major raw material used for the scrap melting process) contains a large amount of zinc in the form of galvanized steel sheet and this zinc has an adverse effect on the refractories as mentioned above (after accumulation in the furnace) and deposits on the inner wall of the exhaust gas duct after discharge together with exhaust gas from the furnace.
The conventional scrap melting process mentioned above is not given any special attention for the disposal of zinc.
It is an object of the present invention to provide a scrap melting process of entirely new type, which, in contrast with the conventional one, is capable of melting scrap and producing molten iron more efficiently. In addition, it offers the following advantages.
Ability to produce a large amount of high-calorie exhaust gas valuable as fuel gas. PA1 More economical operation than the conventional process owing to the utilization of high-calorie exhaust gas. PA1 Ability to produce the high-calorie exhaust gas and/or heat partly from waste plastics. (This permits mass disposal and effective use of waste plastics.) PA1 The fuel ratio is higher than 300 kg/t-pig. PA1 In the case where pulverized coal alone is supplied from the burner, the ratio of the pulverized coal ratio (kg/t-pig) to the top-charged coke ratio (kg/t-pig) is higher than 1.0. PA1 In the case where both pulverized coal and waste plastics are supplied from the burner, the ratio of the sum of the pulverized coal ratio (kg/t-pig) and the waste plastics ratio (kg/t-pig) to the top-charged coke ratio (kg/t-pig) is higher than 1.0.
It is another object of the present invention to provide a new scrap melting process which permits the mass disposal and effective unitization of dust discharged from steel mills and also permits the recovery of zinc (in highly concentrated form) from scrap and dust without it accumulating in the furnace.